The present invention relates to a spark plug suitable for use in internal-combustion engines for automobiles and the like.
Hitherto, there is known a spark plug of this type, as shown in the specification of West Germany Pat. No. 2,256,823, which is improved to be longer in life by joining a heat-resistant and wear-resistant discharging layer made of platinum or the like to the spark discharge end of the central electrode by means of the resistance welding method thereby to minimize the wear of the spark discharge end of the central electrode.
In the conventional spark plug, however, the discharging layer often falls off disadvantageously.
As the result of the examination of the frequent falling off of the discharging layer, the inventor has found that the joint between the discharging layer and the central electrode has been cracked and oxidized, and that the discharging layer has fallen off at the position of this cracked and oxidized portion.
This phenomenon is considered to be largely owing to the thermal stress caused by the difference in linear expansion coefficient between platinum constituting the discharging layer and nickel which is the base metal constituting the central electrode.
More specifically, in the conventional spark plug, it is intended to greatly improve the wear resistance of the electrode by employing the platinum discharging layer as the discharge surface of the central electrode and/or the ground electrode. The discharging layer is, however, constituted by a single chip made of an alloy of platinum and iridium, or an alloy of platinum and tungsten, or an alloy obtained by adding a small amount of nickel to platinum and iridium. The linear expansion coefficient of the alloy is about 8-9.times.10.sup.-6 /.degree.C.; hence, there is a linear expansion coefficient difference of about 5.times.10.sup.-6 /.degree.C. between the discharging layer and the central electrode (and/or the ground electrode). However, the spark plug is used under various operating conditions, high loads and low loads, i.e., the spark plug is alternately and repeatedly subjected to high temperatures and low temperatures. Owing to this alternate repetition of high and low temperatures and the linear expansion coefficient difference, the joint between the discharging layer and the central electrode (and/or the ground electrode) is repeatedly subjected to the thermal stress to generally cause transverse crackings (a) as shown in FIG. 3-A and oxidized portions (b) on the weld surface as shown in FIG. 3-B, resulting in the falling off of the discharging layer 7 unfavorably. It is to be noted that although a platinum chip 9 on the ground electrode 8 is transversely cracked and oxidized, since the ground electrode 8 becomes higher in temperature than the central electrode 6, the platinum chip 9 falls off owing to the wear of this electrode itself. For coping with this situation, hitherto, various methods have been devised. However, none of the methods are effective for engines in which temperature becomes extraordinarily high and for electrode constructions in which the plug electrode becomes high in temperature, e.g., the electrode construction shown in FIG. 4 (in which the central and ground electrodes are projected about 3 to 7 mm more than those of the conventional plug).